i.MX 9 - How to Use the Preempt-RT Kernel in the Standard Yocto Linux BSP

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This article describes how to use the Preempt-RT Linux kernel in the i.MX Linux BSP 6.6.23_2.0.0. This is particularly useful for platforms such as i.MX 95, for which there is not yet a Real-Time Edge Software release.

How to do it

1. Follow the steps in the i.MX Yocto Project User's Guide and build your preferred image, for example core-image-minimal. Will further assume that the BSP is in the ~/imx-yocto-bsp directory and the build directory is ~/imx-yocto-bsp/build.

2. Unpack the attached archive in ~/imx-yocto-bsp/sources. This should create the ~/imx-yocto-bsp/sources/meta-otherkernels directory. This archive will work out of the box for i.MX 95 and i.MX 93, and may require some modifications for other platforms, as described below.

3. Add the meta-otherkernels to your build using the following command:

bitbake-layers add-layer ~/imx-yocto-bsp/sources/meta-otherkernels

4. Add the OVERRIDES .= ":preempt-rt" to ~/imx-yocto-bsp/build/conf/local.conf file using the following command:

echo 'OVERRIDES .= ":preempt-rt"' >> ~/imx-yocto-bsp/build/conf/local.conf

This enables the Preempt-RT kernel for your build. You can always go back to your regular kernel by removing this line from ~/imx-yocto-bsp/build/conf/local.conf.

5. Build again your image. After booting this image, you can check the kernel version using:

uname -a

How it works

The meta-otherkernels layer contains a .bbappend  for the linux-imx kernel recipe which replaces the sources URL with the Real-Time Edge kernel when the "preempt-rt" override is active. In addition, due to the fact that the current real-time kernel does not support all the board configurations, the layer config file (meta-otherkernels/conf/layer.conf) removes from the build the device tree files that are not supported (when "preempt-rt" override is active).

If you use this layer for other SoCs (other than i.MX 93/i.MX 95), you may need to edit the meta-otherkernels/conf/layer.conf and add the unsupported device trees. If an  unsupported device tree is left, Yocto will give an error during build. 

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